In a gastropub in a northern town, a modestly dressed, ordinary-looking woman sits by the door and waits. She takes a sip of wine and scans the room. She is hoping that a gaggle of beauticians are going to burst in at any moment. When they do, she will manoeuvre herself into position and eavesdrop on their conversation. If she is really lucky, she will be able to sidle over, start chatting to the women and persuade them to reveal that their friend is illicitly working while fleecing her boss for maternity benefit - and simultaneously trying to steal his business from under his nose.

The woman is Juliet Motley-Wilcock, the founder of a small company of female private investigators. Across the north Midlands from their base in Grantham, Lincolnshire, Motley-Wilcock and her six colleagues deal with the messy and emotional world of failing relationships, small businesses with a recreant in their midst and middle-aged women targeted by debonair but ruthless conmen. Today she is trying to rumble a fraudster. Her client has had a tip-off that the beautician whom he believes is trying to steal his business is on a girls' night out at this pub and he has sent Motley-Wilcock along to observe. If she proves his suspicions, he can sack his employee, save his business and potentially claim back the maternity pay he gave her.

"I never think of myself as a private investigator," Motley-Wilcock explains. "I think of myself more as a professional friend. I am a personal investigator. I draw clients, even male clients, to me because I am female. They think they are going to get where they want to be through me, far more than a man in a trilby."

In this line of work, the majority of which involves investigating the suspicions of jealous spouses, Motley-Wilcock says women do a better job. "With me, what you see is what you get. I am not into gadgets. I have a 3G phone and a digital camera. That's all I need. Women make better detectives because they are so tuned into the subtleties of people's behaviour."

Most of the time, she says, she tries to persuade the people who call that they don't need her services. "When I get a call, I say to them, 'If you're speaking to me, it sounds like your relationship is already over. You don't need me to give you a photograph.' If their spouse has been unfaithful to them countless times before, what they really need is to make a decision."

In her office, earlier in the day, she had shown me a long-range listening device that can eavesdrop to a distance of several buildings away. She explains how easy it is to bug a room or track someone's location. "I don't supply these things but I can tell them what is available. The tabloids are full of adverts for this stuff. Look in the Express and the Daily Mail and you will find all these things. Everything I do, the clients could do themselves. But they don't want to."

She drives me to the back of a housing estate nearby, to a small opening in a hedge. Recently, Motley-Wilcock was in this spot for a client. She had a call from a married woman who believed her husband was having an affair. She sent Motley-Wilcock to follow her husband as he left his house at the wealthier end of the estate and used a short cut, only accessible through an opening in a hedge, to visit his lover on the other estate. She snapped a picture as he greeted his mistress and passed on the evidence to the man's wife.

"I get a lot of work on new housing estates," she says. "It seems that people move to a new place to start afresh, but then whatever is going wrong obviously doesn't seem to get better. It's strange."

Motley-Wilcock arrived at her career by accident. Five years ago she was divorced, and while waiting to receive her financial settlement she began a relationship with a conman. She ended up handing over £75,000, her entire financial settlement, over the course of a year. "I just obeyed. I gave him everything."

She began to look up information about him and his financial history. "I ended up with a big file on him, his financial history, company details. It's astounding what you can find on someone over the internet. I photocopied it three times and put it in three different locations." She showed copies to her solicitors but there was nothing they could do. "It was then that friends offered to watch him for me, to see what he was up to. But who could I trust to do that? I felt so stupid."

She was already working as a life coach, and knew from her current clients that there was a lot of potential work for an investigator. She did a course in private investigation and then advertised in the Yellow Pages.

Since that time she has trawled the housing estates of the east Midlands exposing the unfaithful and helping the cuckolded through the ends of their marriages. "It is very rewarding work, and not all of it ends unhappily," she says. "I had a very rich male client once. He was engaged to marry a very attractive woman much younger than him and was a little insecure about it. His fiancee used to go on these chill-out weekends on her own, at a health spa. He phoned me and asked me to go along too, in case she was up to something. I followed her all weekend, and got chatting to her over dinner one night. She loved him rotten and I saw nothing at all untoward. My client got great peace of mind and went ahead and married knowing he was loved.

"I had another client who suspected his wife was going away for 'clothing optional weekends' to have sex with other men. He asked me to go along and observe her. You were allowed to wear briefs if you wanted to but it was frowned upon so to blend in, I had to go naked. I saw his wife there, up to no good, but in the end they stayed together. He just wanted to know. People are strange. They think they want to know but really they don't mind."

Though she finds the resolution of a case rewarding, Motley-Wilcock says she doesn't enjoy the actual investigation work because of the hours sitting around in disguise. She has a large sports bag crammed full of changes of clothes that sits in the back of her car at all times. "You know if someone has noticed you and you have to go and change," she explains, rifling through her bag. "I am 47, and some days I wear a big coat and bulk myself up so I look older and blend in. I sometimes put on a blonde wig and heels and drive my sports car if I need to be young. If I am concealing myself in a park or something, I hide buttons and jewellery so there are no shiny surfaces that can reflect light and reveal me. If I am asked any questions, I can say I am looking for my dog."

Anyone, in fact, can turn detective if they have an instinct that something is up. "Christmas is a big time for bust-ups," she says. "If you think your partner has taken up with someone else, you can go with a friend and carol sing at the house you think he is at. Go early before they have sloped off to bed together."

Back at the gastropub, we are nearly at the end of our second glass of wine and our target has still not shown up. Tomorrow, Motley-Wilcock will report in to her client and come up with a plan B, but for now she decides to relax. After all, tomorrow she could be anywhere.